A potassium fertilizer field trial of rain-fed cotton crop was carried out on ferrasols at two sites in Benin. The measurements focused on leaf formation patterns and their ability to convert intercepted photosynthetically active radiation (PAR) into biomass. The results highlighted the role of K in plant architecture and leaf area development. On K deficient plots, the reduction in leaf area was mostly the result of a decreased number of leaves and, to a lesser extent, leaves of a smaller individual size. During vegetative growth, leaf senescence did not differ between treatments. Biomass partitioning to different plant compartments was found to be affected by potassium deficiency, in favour of leaves. No significant effect of K treatment was observed on the conversion of intercepted radiation into biomass.